1. Field of the Invention
The present invention relates to a micromotion mechanism having an ultrasonic motor and a microscope apparatus having the micromotion mechanism.
2. Description of the Related Art
A microscope is widely used in observing the microstructure of a semiconductor, a living body sample, etc. An XY stage is used in setting an optional position of an observation target for a microscopic observation. In this case, a feed resolution demanded for a microstructure to be observed and the stability in a static position are required. In addition, it is often necessary to observe plural positions of an observation object with high throughput and at a high speed.
One of the actuators corresponding to the above-mentioned requests is an ultrasonic motor. For example, as described in the patent document 1 (Japanese Published Patent Application No. 2005-265996), there is an apparatus proposed using an ultrasonic motor as an actuator of the XY stage for a microscope.
An example of the ultrasonic motor used for this stage is a rectangular parallelepiped linear driving ultrasonic actuator. Most of these ultrasonic motors is configured by a multiplayer piezoelectric element, includes a electrode for flexural vibrations and a electrode for longitudinal vibrations, and is driven by applying sine wave signals 90° shifted to the respective electrodes.
FIG. 1 is a schematic diagram showing an example of a stage translation mechanism provided with the above-mentioned linear driving ultrasonic actuator.
In the example shown in FIG. 1, an ultrasonic motor 106 has an ultrasonic vibration element (hereinafter referred to simply as a vibration element) 105 including a multiplayer piezoelectric element having flexural vibration electrodes 101 (101a, 101b, 101c, 101d) and a longitudinal vibration electrode 102 and two drive elements 104 (104a, 104b). When a longitudinal vibration signal as a sine wave signal is applied to the longitudinal vibration electrode 102, and a flexural vibration signal as a sine wave signal 90° shifted from the longitudinal vibration signal is applied to the flexural vibration electrode 101, a movable element (stage) 108 moves along a guide 107.
In FIG. 1, the “+” or “−” sign of the flexural vibration electrode 101 and the longitudinal vibration electrode 102 indicate the polarization direction of the piezoelectric element. For example, when a plus voltage is applied to an electrode, a piezoelectric element of an electrode portion of a “+” sign deforms to be expanded in the longitudinal direction, and a piezoelectric element of an electrode portion of a “−” sign deforms to be reduced in the longitudinal direction. Therefore, when a sine wave signal is applied to the flexural vibration electrode 101, a flexural deforming vibration as shown by the schematic diagram in FIG. 2A is excited, and when a sine wave signal is applied to the longitudinal vibration electrode 102, a longitudinal vibration of expansion and reduction in the longitudinal direction as shown by the schematic diagram in FIG. 2B is excited. In FIGS. 2A and 2B, an arrow expressed by dotted lines indicates the direction of the deformation of the piezoelectric element. An arrow expressed by solid lines indicates the direction of the movement of the drive element 104. Thus, when the phases of two types of vibration are simultaneously excited with 90° shifted from each other, the drive element 104 vibrates to draw a locus of an oval (refer to the dotted lines shown in FIG. 1) as indicated by the arrow shown in FIG. 1. At this time, the friction can be reduced by the vertical component of the force generated when the drive element 104 touched the movable element 108, and the force of the horizontal component moves the movable element 108.
Recently, the observation magnification has been higher by the super-microstructure of an object to be observed in observing the line width of a semiconductor, a living body, etc., and drive resolution is required for a submicron order in the micromotion mechanism in which an observation sample is positioned.
To meet the above-mentioned needs, for example, the patent document 2 (Japanese Published Patent Application No. 2001-161081) proposes a driving method using a signal and intermittently having a burst waveform portion as a method of enhancing the precision in stop position by improving the drive resolution of an ultrasonic motor. However, when such signal is applied to an ultrasonic actuator, noise like that generated by metal occurs at the start (activation) and the end (stop) of a burst waveform portion (111 and 112) as shown by the schematic diagram in FIG. 3.
To suppress the generation of the noise, for example, the non-patent document 1 (“Guide to Precise Control of Actuator” edited by Solid-state Actuator Study Group of Japan Technology Transfer Association, p. 598-p. 601) proposes a method of long-time stepwise or continuously increasing or decreasing an amplitude until a waveform indicates the maximum amplitude or the amplitude reaches 0 as shown in FIG. 4 when a burst waveform signal is applied.
To perform micromotion drive using the above-mentioned burst waveform signal, it is necessary to reduce the number of burst waveforms. In the meantime, to prevent the noise from occurring, a burst waveform signal is to be applied for a predetermined time (several millisecond order). In the experiment performed by the Applicant of the subject patent application (hereinafter referred to simply as the “subject application”), at the start and end of the burst waveform signal, it is necessary to suppress the noise to gradually change the amplitude of the burst waveform signal respectively for 2 milliseconds or more. In this experiment, the frequency of the burst waveform signal is about 80 kHz. In this case, as indicated by the schematic diagram shown in FIG. 5, the total application time of the burst waveform signal is 4 (2+2) milliseconds by assuming that the number of waveforms at the maximum amplitude is 1. Thus, the number of burst waveforms is 300 or more. In addition, the amount of movement of the movable element is about 2 μm at this time.